Wasps hurl native ants away from food

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Introduced wasps competing with native ants for protein-rich foods have been filmed lifting up and carrying away ants which get too close to the food.

New Zealand researchers said today the behaviour gaves only short-term benefits to the wasps, but was likely to be aimed at stopping the first ant "scouts" from finding the food and bringing larger numbers of ants.

Researchers Julien Gangier and Philip Lester, of Victoria University, filmed the wasps in South Island beech forests where baits of canned tuna had been laid, and said today a widespread behaviour of the wasps was to fly off with an ant and drop it away from the food.

"Particular attention was paid when a wasp removed an ant by flying backward and dropping it away," they said.

The wasps increased their competitive efforts when the ants were more numerous, the pair said today in the UK Royal Society's journal, Biology Letters.

"They dropped their competitors away even when bait stations were frequented by several tens of ants," the researchers said.

Nearly 90 per cent of the removed ants were not injured, even though in some cases the wasps literally hurled the ants away.

The lack of killing by the wasps surprised the researchers, who said that a chemical excreted by workers among the small brown bush ants was "clearly repellent" for the wasps.

At three sites wasps picked up an ant feeding or walking around the food, flew off several centimetres and dropped it, then returned to the food.

The study extended knowledge of the "interference" behaviour the "social" wasps (scientifically known as vespula vulgaris) used, in this case against the aggressive native ant prolasius advenus.

Both species fed on carbohydrate-rich "honeydew" excreted by sapsuckers on beech trees, but their populations were limited by access to protein rich foods. They have been seen competing for the corpses of dead cicadas.

The ant-dropping behaviour was also observed under natural conditions between a wasp and a group of ants foraging in leaf litter for mealworms.

Few scientific observations have been made of wasps guarding food against ants, and the researchers expected that knowledge of the underlying behavioural mechanisms could be of crucial importance for understanding of biological invasions involving the two social insect groups.

Similar food fights between native ants and invasive wasps have been suggested as a potential explanation for the relatively low success of another pest wasp, vespula germanica in Argentina.

- NZPA